Optical emission spectroscopy (OES) is a technique for the elemental analysis of samples, known also as Atomic Emission Spectroscopy (AES). OES uses the intensity of light at a particular wavelength emitted from a sample subjected to, for example, a flame, plasma, arc, or spark to determine the quantity of an element in a sample. Light is emitted by the excited atoms and ions of the elements of the sample as transitions occur from an excited state to a lower energy state. Each element emits light of discrete wavelengths characteristic of its electronic structure, which are also termed spectral lines. By separating and detecting the spectral lines, OES can provide a qualitative and quantitative determination of the elemental composition of the sample. The spectrometer of the present invention is particularly suitable for so-called spark OES, which is useful, for example, in the analysis of solid metallic samples. In spark OES, an electrical discharge, such as a condensed arc or spark for example, is used to rapidly vaporise a solid sample and excite elements in the vapourised sample. A spark OES spectrometer includes a spark stand or chamber for ablating the sample material and exciting the elements in the sample to emit light, an optical system for dispersing the emitted light into discrete wavelengths and a detection system for detecting the intensity of the dispersed light. Furthermore, the spectrometer typically comprises a data processing and storage system for processing and storing signals from the detection system, e.g. representing the light intensity. To build up sufficient data for determination of the composition, a succession of sparks is typically employed and the resulting data generated from the sparks is accumulated for processing.
A known type of spectrometer optics for OES is the flat field spectrometer in which the dispersed light is imaged substantially linearly at one or more detectors over the spectral range of interest. This enables the use of a flat surface detector, typically a charge coupled device (CCD). A flat field spectrometer is particularly suitable for use with a linear CCD detector. Double or triple flat field spectrometers may be constructed in which two or three separate gratings may be used, each of which receive light from the sample through their own respective entrance slit. The separate gratings each form a separate spectrum in a different spectral range on its own respective detector. Such double or triple flat field spectrometers are thus more bulky than single flat field spectrometers since they respectively require two or three entrance slits, two or three gratings and two or three detectors. In such systems, each entrance slit requires its own viewing angle of the sample plasma, which must be accommodated.
A compact double flat field spectrometer is disclosed in WO 2011/098726. In this document, instead of using one entrance slit per grating and a separate detector, a flat field spectrometer is described which comprises only one entrance slit but two diffraction gratings and one detector having a plurality of lines of photodetectors. The single detector is thus an array detector. Each grating diffracts a portion of the light received through the entrance slit and each grating forms a spectrum on a separate line of the array detector. In this way, a double flat field spectrometer is constructed that is very compact and low cost whilst covering a relatively broad spectral range. The present invention is particularly, although not exclusively, applicable to a compact double flat field spectrometer as disclosed in WO 2011/098726. In fact, both the flat field spectrometer disclosed in WO 2011/098726 and the present invention can be used with more than two gratings. For example, a compact flat field spectrometer is disclosed in WO 2011/098726 having four diffraction gratings that each receive light through the single entrance slit.
A problem with OES, which is not addressed by the spectrometer disclosed in WO 2011/098726, is that of spectral interference. This is where a spectral line of analytical interest is interfered with by another spectral line at similar or the same wavelength as the line of analytical interest. In this way, the partial or complete overlap of the spectral lines means that it is difficult to extract information from the line of analytical interest.
A further challenge in an optical arrangement for spark OES is to pass towards the gratings and detector as much as possible of the light that is analytically needed and/or to reduce the high excitation energy background, which is emitted near the sample surface. It is another challenge to illuminate constantly the grating in order to obtain a constant resolving power.
Against this background, the present invention has been made.